Question: The house Trevor's family lives in has $6$ people (including Trevor) and $3$ bathrooms. In the past month, each person showered for an average of $480$ minutes and used an average $72$ liters of shower water (over the entire month). Water costs $0.20$ dollars per liter. How much money did Trevor's family spend, in total, on shower water in the past month?
There can be many ways to solve this problem. Here, we will do this by thinking about units. Let's say that last month, Trevor's family spent $x\,\text{dollars}$ on water. We are given that the price of water is $0.20\,\dfrac{\text{dollars}}{\text{liter}}$. How can we relate these two quantities with an equation? $\begin{aligned} y\,\text{liters}\cdot 0.2\,\dfrac{\text{dollars}}{\text{liter}}=x\,\text{dollars} \end{aligned}$ So in order to find the total cost $x$, we need to figure out the value of $y$, which is the number of liters of water Trevor's family used. Notice what other information we are given: $6\,\text{persons}$ $3\,\text{bathrooms}$ $480\,\dfrac{\text{minutes}}{\text{person}}$ $72\,\dfrac{\text{liters}}{\text{person}}$ Which of these quantities can help us calculate a quantity whose units are $\text{liters}$ ? We can combine the following quantities: $\begin{aligned} 6\,\cancel\text{persons}\cdot 72\,\dfrac{\text{liters}}{\cancel\text{person}}=432\,\text{liters} \end{aligned}$ Now we can plug that in the original equation: $\begin{aligned} 432\,\text{liters}\cdot 0.2\,\dfrac{\text{dollars}}{\text{liter}}&=x\,\text{dollars} \\\\ 86.4\,\text{dollars}&=x\,\text{dollars} \end{aligned}$ In conclusion, Trevor's family spent $86.40$ dollars for shower water in the past month.